1. Field of the Invention
The present invention relates to a high-frequency filter, and more particularly to a dielectric filter, a dielectric duplexer, and a communication device, which are used in a base station of a microwave band communication system.
2. Description of the Related Art
A first example of a conventional dielectric filter will be described with reference to FIG. 1.
The dielectric filter 110 is constituted of two dielectric resonators 120a, 120b arranged in parallel, and metallic panels 111a, 111b for covering the opening parts of the dielectric resonators 120a, 120b. Each of the dielectric resonators 120a, 120b is constituted of a rectangular-prism-shaped cavity 121 made of a dielectric ceramic, and a dielectric block 122 disposed within the cavity 121. A conductive layer 123 is formed by painting and baking a silver paste on an outside surface of the cavity 121. The dielectric block 122 has a cross-shape in which two dielectric poles are intersected. Typically, the cavity 121 and the cross-shaped dielectric resonator 122 are integrally molded. Coupling loops 112a, 112b are mounted to the metallic panel 111a. One end of each loop is connected to a central conductor of a coaxial connector 113a, 113b mounted to the metallic panel 111a, and the other end thereof is grounded by being connected to the metallic panel 111a. Further, a coupling loop 112c for electromagnetic-coupling the two dielectric resonators 120a, 120b is mounted to the other metallic panel 111b. 
When a signal is inputted from an outside, a magnetic field is generated in the area surrounding the loop 112a, and the generated magnetic field couples to a magnetic field surrounding one of the dielectric poles in the dielectric block 122. Further, an electromagnetic field around the one of the dielectric poles and an electromagnetic field around the other one of the dielectric poles that is perpendicular thereto are coupled by a groove 125 formed at the intersection of the two dielectric poles of the dielectric block 122. For the other dielectric resonator 120b, a similar chain of electromagnetic field couplings occurs, and as a result, the dielectric filter 110 functions as a fourth order band pass filter.
The loop 112a is constituted of a first part 112a1 that extends in a direction that is the same as a length direction of one of the dielectric poles, and a second part 112a2 that extends in a direction perpendicular to the first part 112a1. The loop 112b has a similar structure. Consequently, the first part 112a1 of the loop 112a couples to one of the dielectric poles extending in the same direction of the dielectric block 122, and at the same time the second part 112a2 of the loop 112a couples to the other one of the dielectric poles in the dielectric block 122. As such, it is possible to provide an attenuation pole on either a low frequency side or a high frequency side of the resonant frequency of the dielectric resonator, by electromagnetic-coupling the loop 112a to the first and second resonators formed by the respective dielectric poles in the dielectric resonator 120a simultaneously.
In general, for a signal with a frequency lower than a resonant frequency, its phase will not change even when passing through a resonator, but for a signal with a frequency higher than the resonant frequency, its phase will change by xcfx80 when passing through the resonator. For example, when coupling to an in-phase resonant mode that occurs in one of the dielectric poles corresponding to the first part 112a1 of the loop 112a, and coupling to a reversed-phase resonant mode that occurs in the other one of the dielectric poles corresponding to the second part 112a2 of the loop 112a, an attenuation pole is generated on the low frequency side of the resonant frequency similarly.
A second example of a conventional dielectric filter will be described with reference to FIG. 2. FIG. 2 is a perspective view of a dielectric filter according to a second conventional example. Moreover, the identical symbols are attached to the same parts as in the previous conventional example, and it will be illustrated by showing only the dielectric resonator that constitutes the dielectric filter.
In the conventional dielectric resonator 120c shown in FIG. 2, dent parts 124 are formed in an outside of the cavity 121 extending toward an inside thereof, at the four points where the cross-shaped dielectric resonator 122 joins the cavity 121. As a result, the dielectric resonator 120c has three resonant modes, i.e., TM110 mode, TM111 mode, and TM110 mode at respective parts thereof, similar to those shown in the electric field distribution diagram of FIG. 4, and the dielectric filter functions as a three-stage band pass filter.
Because several spurious modes are generated outside the pass band, it is necessary in a dielectric filter used in a communication base station, and the like, to provide attenuation poles both on the low frequency side and on the high frequency side of the pass band in order to restrain them. However, in the dielectric filter in the first conventional example, with the dielectric resonator having two resonant modes, and the loop providing input/output (I/O) coupling for these two resonant modes simultaneously, an attenuation pole can be provided on either the low frequency side or the high frequency side, but not both. Accordingly, in order to provide the attenuation poles both on the high frequency side and on the low frequency side, it is necessary to arrange one additional dielectric resonator in parallel, in order to provide the attenuation pole on the other side. That is, in the first conventional example, for providing the attenuation poles on the low frequency side and on the high frequency side, two dielectric resonators are always required, and thus there is a problem of increasing the size of the dielectric filter.
Further, in the dielectric filter in the second conventional example, there is no way to provide attenuation poles on both the low side and the high side of the resonant frequency band.
Accordingly, it is an object of the present invention to provide a dielectric filter, which solves these problems, which provides attenuation poles on the low frequency side and on the high frequency side of the resonant frequency, and which is capable of being miniaturized.
These and other objects of the present invention can be achieved by a dielectric filter according to a first aspect of the invention, including a dielectric resonator having at least three resonant modes, and configured including a conductive cavity, and a dielectric resonator arranged within the cavity, and an input/output coupling unit that couples to the dielectric resonator, wherein the input/output coupling unit couples to a resonant mode at the first stage, as well as to at least one resonant mode at the h-th stage (h=2n+1: where n is an integer) approximately in negative-phase with respect to the first stage, among the resonant modes of the dielectric resonator.
The objects of the present invention can also be achieved by a dielectric filter according to a second aspect of the invention, including a dielectric resonator having at least three resonant modes, and configured including a conductive cavity, and a dielectric resonator arranged within the cavity and an input/output coupling unit that couples to the dielectric resonator, wherein the input/output coupling unit couples to a resonant mode at the last stage, as well as to at least one resonant mode at the (kxe2x88x922n)-th stage (where n is an integer), the last stage being the k-th stage, approximately in negative-phase with respect to the last stage, among the resonant modes of the dielectric resonator.
In the dielectric filter according to the foregoing first and second aspects of the invention, preferably the input/output coupling unit is a loop having conductivity, and the input/output coupling unit is arranged in a direction such that it is coupled approximately in negative-phase with respect to a resonant mode to which the input/output coupling unit couples.
It is another object of the present invention to provide a dielectric duplexer which solves these problems, which provides attenuation poles on the low frequency side and on the high frequency side of the resonant frequency, and which is capable of being miniaturized.
The object of the present invention can also be achieved by a dielectric duplexer, including at least two dielectric filters, an input/output coupling unit coupling to each of the dielectric filters, respectively, and a unit for use in connecting to an antenna that is commonly connected to the dielectric filters, wherein at least one of the dielectric filters is a dielectric filter according to either one of the first and second aspects of the invention.
In the dielectric duplexer described above, it is preferable that the input/output coupling unit is a loop having conductivity, and each input/output coupling unit is arranged in a direction such that it is coupled approximately in negative-phase with respect to a resonant mode of the corresponding dielectric resonator to which the input/output coupling unit couples.
It is another object of the present invention to provide a communication device which solves these problems, which provides attenuation poles on the low frequency side and on the high frequency side of the resonant frequency, and which is capable of being miniaturized.
The object of the present invention can be achieved by a communication device, including a dielectric duplexer, a circuit for use in transmitting that is connected to one of the input/output coupling units of the dielectric duplexer, a circuit for use in receiving that is connected to another one of the input/output coupling units, and an antenna for being connected to the unit for use in connecting to an antenna of the dielectric duplexer, and including the dielectric duplexer and preferably the input/output coupling units described above.
For a signal with a frequency lower than a resonant frequency, its phase does not change when passing through a resonator, but for a signal with a frequency higher than the resonant frequency, its phase changes by xcfx80 as it passes through the resonator. Accordingly, when passing through a route such as a first stage, a second stage, a third stage, and so on, sequentially, at the h-th stage the signal is as if it has passed through the even numbered resonator, so that the phase of a signal in the resonant mode at the h-th stage is in-phase with the signal at the coupling location at the first stage for both a signal with a frequency lower than the resonant frequency and a signal with a frequency higher than the resonant frequency. On the other hand, in the other one of the routes, i.e., a route having coupling to the resonant mode at the h-th stage directly from the input/output coupling unit, the signal at the h-th stage is in negative-phase with respect to the phase of the signal at the first stage. That is, according to the dielectric filter of the present invention, the signals on the low frequency side and on the high frequency side of the resonant frequency are in negative-phase at the h-th stage, and thus it is possible to provide attenuation poles on the low frequency side and on the high frequency side of the resonant frequency with one dielectric resonator.
In another arrangement, which is similar to the previous one, at the last stage the signals on the low frequency side and on the high frequency side of the resonant frequency are in negative-phase, thereby making it possible to provide attenuation poles on the low frequency side and on the high frequency side of the resonant frequency with one dielectric resonator. Accordingly, by combining the dielectric filters as described above, it is possible to provide two or more attenuation poles on the low frequency side and on the high frequency side of the resonant frequency, respectively.
Furthermore, the dielectric filter according to the present invention is preferably such that the input/output coupling unit is a loop having conductivity, and the input/output coupling unit is arranged in a direction such that it is coupled in an approximately negative-phase with respect to a resonant mode to which the input/output coupling unit couples.
As a result, by changing only the arrangement direction of the loop, it is possible to couple to the resonant modes at the first and the h-th stages, or to the resonant modes at the last and the (kxe2x88x922n) stages in negative-phase, respectively.
Moreover, the dielectric duplexer of the present invention includes at least two dielectric filters, an input/output coupling unit coupling to each of the dielectric filters, respectively, and a unit for use in connecting to an antenna that is commonly connected to the dielectric filters, wherein at least one of the dielectric filters is a dielectric filter as described above.
Furthermore, the communication device of the present invention includes a dielectric duplexer as described above, a circuit for use in transmitting that is connected to at least one of the input/output coupling units of the dielectric duplexer, a circuit for use in receiving that is connected to at least another one of the input/output coupling units, and an antenna for being connected to a unit for use in connecting to an antenna of the dielectric duplexer.
As a result, attenuation poles are provided on the low frequency side and on the high frequency side of the band, thereby enabling the dielectric duplexer, and the communication device, to have excellent characteristics.
Other features and advantages of the invention will be understood from the following description of embodiments thereof, in conjunction with the drawings, in which like references denote like elements and parts.